1. Field of the Invention
The present invention relates to a color image encoding method for a color facsimile etc. for communication of color image.
2. Related Background Art
In the conventional image encoding method, a run length encoding system represented by G3, G4 facsimile recommended by CCITT (Consultative Council of International Telephone and Telegraph) has been generally used. In this encoding system the length of continuation (run length) of white or black pixels is counted and in reference to the predetermined code table, the code corresponding to the counted value is determined. The encoding table used here is so characterized that a relatively short code is allotted to the long white run which often appears in the case of a document image.
On the other hand, low cost color printers have been developed in recent years and an image communication system with binary color having 1 bit data for each of red (R), green (G), blue (B) or yellow (Y), cyan (C) and magenta (M) has been introduced.
One such binary color encoding system, can perform encoding for each of the 3-color bit planes and can use one MH and MR encoding method for black & white picture.
FIG. 1 is an example of the image transmission device to conduct encoding for each 3-color bit plane. In the buffer memories 90-a, b and c are stored, color by color, the R, G and B color signals binarized by a color image reading device not shown in the figure.
First the R signal 300 is input into the encoder 93 from R buffer memory 90-a through selector 98. At the encoder 93, MH and MR codes etc. are encoded and R code 94 is generated. This R code 94 is decoded at decoder 97 and via the selector 99 it is input into R buffer memory 91-a. When encoding of the R signal is over, the G signal 301 is output from G buffer memory 90-b in the same way and then through the similar encoding processing, a G code 95 is generated and, through the similar decoding processing, decoded data are stored in G buffer memory 91-b. In the same way, B code 96 is produced by the B signal coming from B buffer memory 90-c and it is encoded and stored in B buffer memory 91-c.
At printer 92, all data are accumulated in buffer memory 91-a, b and c and thereafter each color data is output and recorded. Or alternatively, it may be so arranged that after accumulating all the data in buffer memory 91-a and 91-b, the B signal is obtained by decoding the B code and is recorded first at printer 92 and then the color data of 91-a and 91-b are recorded.
However, when such a method as to encode each color of a 3-color bit plane, is used the code of each color is independently produced. Therefore, even for a printer which can simultaneously record three colors, such as an ink jet printer, it is necessary that the decoding device is equipped with a buffer memory corresponding to three or two colors.
This method also presents the following drawback: recording can be started only after 3- or 2-color image data are decoded, which includes promptness in data transmission.
Individual encoding of the R, G and B bit planes also increases the entropy of the original R, G, and B data source and, as a result, encoding efficiency deteriorates. To put it in another way, it does not utilize the correlation data of colors.
Another problem is that when the statistical character of the black & white run length of the image is different from that of an image which has been used as the standard when the code table was prepared, for example when a semi-intermediate tone image is transmitted, on-off of color dots is frequently repeated. As a result, the amount of codes may finally exceed the original data.
This method also decreases efficiency because when an image is encoded where black & white images are mixed into color images, R, G and B have the same value. Therefore, even the black & white images which can be expressed in monocolor, must be encoded for each of the R, G and B bit plane.
In other words, with the system to encode color images in 3 bits, when
(1) the input manuscript is only in a few colors out of 8 available colors such as white, black and red or white, blue and green, or when PA1 (2) the printer at the signal receiving side can express only a few colors out of the eight colors, then the system encodes unnecessary color data and thus data compression becomes insufficient.